This invention relates to communication systems and more particularly to acquisition of communication systems by mobile terminals.
Public wireless radiotelephone systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a conventional terrestrial wireless public communication system 20 that may implement one of the aforementioned wireless communication standards. The wireless system may include one or more wireless mobile terminals 22 that communicate within a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between wireless mobile terminals 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. Through the communication system 20, a duplex radio communication link 30 may be effected between two wireless mobile terminals 22 or between a wireless mobile terminal 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications within the cell 24 to and from the wireless mobile terminal 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals.
As mobile terminals in cellular radio communication systems may cross geographic regions served by different base stations, it is known in the prior art that means should be provided to select the appropriate base station to service a mobile terminal. In the U.S. AMPS system, the mobile terminal would typically use signal strength measurements (such as RSSI) to determine the strongest base station to which it should listen for calls during standby (idle) mode. This determination of which base station to communicate with may be referred to as site scanning (reselection) as periodically the radiotelephone determines the RSSI for other base station sites. Also in the U.S. AMPS system, base stations belonging to the cellular network typically listen to the signal strengths received from mobile telephones that are actively transmitting during calls, and the network uses its measurements to determine an optimum base station for handling a call in progress. When a call in progress is switched from one base station to another, it is commonly known as xe2x80x9chandoverxe2x80x9d or xe2x80x9chandoff.xe2x80x9d
Cellular telephones using a Time Division Multiple Access (TDMA) method conforming to the TDMA cellular standards, for example, those known respectively as GSM, D-AMPS, IS54, IS136 or PCS1900, may use spare time between transmit and receive timeslots to change frequency and monitor the signal strengths of other base stations. The mobile telephone may make measurements of the signal strengths received from surrounding base stations. Mobile Assisted Handover (MAHO) may be implemented using these measurements. The measurements are typically reported to the currently serving base station, which determines if a handoff or reselection should be made to another base station.
The control channel as described above is a dedicated channel used for transmitting cell identification and paging information often referred to as a broadcast control channel (BCCH). Identification information in, for example, an IS-136 system may be referred to as a Public Service Profile (PSP) and may include frequency information on various channels as well as the cell""s associated Digital Voice Color Code (DVCC), System Identification (SID), System Operator Code (SOC) and Mobile Country Code (MCC). The traffic channels carry the voice and data information. The dedicated control channels are generally broadcast using a different radio frequency in each cell to allow frequency reuse for greater system capacity while limiting the problems caused by interference. The traffic channels are also typically allocated among a plurality of radio frequencies to reduce and preferably eliminate interference using conventional frequency allocation techniques that are well known to those having skill in the art. However, capacity problems may still be encountered, such as in high population density areas.
Capacity limitations and cost and service considerations have been addressed by using private wireless communication systems, which use low power cellular transmissions that provide coverage over a smaller area. These private communication systems may be located within the coverage area of a public communication system such as one of the cellular systems described above. An indoor cellular communication system typically includes one or more mobile stations or units, one or more base stations, a radio control interface, and a mobile switching center. A typical indoor cellular network may have one or more cells.
It is desirable for mobile terminals having access rights to both public and private communication systems to determine the availability of both systems when in operation. Accordingly, it is known, for example under the IS-136, Rev. A standard, to provide mobile terminals which monitor not only for alternate base stations within the public network but also to monitor for transmissions from private communication systems. It is also known to provide transmission of identification information from private communication systems for use by the mobile terminal in its monitoring operations.
Under the IS-136 standard, the private communication system transmits identification information referred to as Private Operating Frequencies (POF). The POFs retained by a mobile terminal are a set of private communication system control frequencies previously identified by the mobile terminal as being actively in use by a private communication system within the vicinity of a public cell (associated by the mobile terminal with a specific control channel frequency) for which the mobile terminal has retained a PSP. Upon acquisition of a particular control channel (CCH) transmission, the mobile terminal may recognize the control channel as corresponding to a PSP retained by the mobile terminal and declare a PSP match. When a PSP match is declared, any stored associated POFs are treated as reselection candidates for the mobile terminal. If the mobile terminal then finds a satisfactory POF candidate (for example, with acceptable signal strength), the mobile terminal typically registers for service on the private communication network (which has an associated private system identification or PSID) and switches to a non-public mode of operation.
A mobile terminal supporting PSP/POF pursuant to the IS-136 Rev. A. standard typically stores at least 4 PSPs and 4 POFs per private communication system (PSID) as shown in Table 1 below. Each private system has an associated identifier (PSID). The PSP/POF information for each PSID is typically updated by information transmitted by the private system on its control channel. Accordingly, the mobile terminal generally must be locked on to the private system""s control channel to be updated if the POF information changes. Furthermore, under IS-136, one of the POFs is the currently used control channels for the private system.
One problem with this approach to integrating private and public systems is that private communication systems may operate within the same frequency spectrum as the surrounding public system and/or adjacent private communication systems. Accordingly, to overcome interference problems, private communication systems may be provided with adaptive channel allocation (ACA) or other automated frequency reallocation means. Therefore, private communication systems may retune on occasion, i.e., change the frequency used for the control channel (or channels in some cases) transmission by the private communication system.
After a retune, the PSP/POF typically must be updated in the mobile terminals. However, for mobile terminals that are not receiving transmissions at the time of the retune, the change will not be received and the mobile terminal will not have a correct POF for the PSP match (i.e. for the public region in which the private system is located). Therefore, the mobile terminal will not be expected to find and camp on the private system automatically. Under the IS-136 standard, this problem is addressed by requiring the user of the mobile terminal to force a non-public search by the mobile terminal to find the private communication system.
In light of the above discussion, improvements are needed in integration of private and public communication systems to provide improved user perception of the ease of accessing either type of system.
It is, therefore, an objective of the present invention to resolve the problem of integration of public communication systems with communication systems subject to retuning.
It is a further objective of the present invention to provide systems and methods which provide users with mobile terminals which automatically track retuning of communication systems without requiring user intervention.
These and other objects are provided, according to the invention, by providing methods and systems for tracking of a communication system allowing operation of a mobile terminal with both a public communication system and the private communication system even when the private communication system retunes its control channel whether or not the mobile terminal is in communication with the private system at that time. This automatic selection of the new control channel for the private communication system is provided by communication of both the current control channel frequency and one or more candidate control channel frequencies which will be used in turn by the private radio communication system in retunes. These candidate frequencies are originally provided to the mobile terminal, for example, when it first establishes a connection with the private communication system. The candidate control channel frequency information is updated during periods when the mobile terminal is connected to the private communication system. When the mobile terminal subsequently leaves the coverage area of the private communication system and then returns to find that the control channel previously used is no longer in use, it attempts to establish a connection using the candidate control channel frequencies. Accordingly, the mobile terminal is able to respond to retunes occurring while it is not in communication with the private communication system without the need for user intervention.
In particular, a method for tracking of a private communication system is provided. A first control channel frequency for the private communication system is selected as well as a candidate control channel frequency. Control information is transmitted on the first control channel frequency which includes an identification of the candidate control channel frequency and an identification of the private communication system. The private communication system may be a TDMA system such as an IS-136 compliant system. The private communication system is preferably within a cell of a public communication system and the control information then also includes identification information for the public cell. In one embodiment of the present invention, the identification information for the cell is a PSP for the cell.
In another embodiment of the methods of the present invention, after the control information is transmitted at the first control channel frequency, the private communication system is retuned to transmit control information on the candidate control channel frequency instead of the first control channel frequency. The control information is then transmitted on the candidate control channel frequency. The control information then includes an identification of a second selected candidate control channel frequency and an identification of the private communication system. Preferably, retuning is performed using an adaptive channel allocation scheme having a determined sequence of alternate candidate carriers for retuning.
In a further embodiment of the methods of the present invention, a plurality of candidate control channel frequencies are selected for the private communication system. Identifications of the plurality of candidate control channel frequencies are transmitted in the control information.
In another aspect of the methods of the present invention, the control information is received by a mobile terminal on the first control channel frequency from the private communication system. The mobile terminal subsequently receives control information from the private communication system on the candidate control channel frequency if control information from the private communication system is absent from the first control channel frequency. The private communication system is retuned to transmit control information on the candidate control channel frequency instead of the first control channel frequency. The control information transmitted on the candidate control channel frequency includes an identification of a second selected candidate control channel frequency and an identification of the private communication system. The mobile terminal then receives control information from the private communication system on the second selected candidate control channel frequency if control information from the private communication system is not being received on either the first control channel frequency or the candidate control channel frequency.
In a system aspect of the present invention, a private communication system is provided. The system includes a means for selecting a first control channel frequency for a control channel and at least one candidate control channel frequency. The system further includes a means coupled to the means for selecting for changing a frequency on which the private communication system transmits the control channel from the first control channel frequency to a selected one of the at least one candidate control channel frequency. The control information is transmitted by a means coupled to the means for changing a frequency on which the private communication system transmits for transmitting control information on the control channel. The control information includes an identification of the at least one candidate control channel frequency and an identification of the private communication system. The private communication system may be an IS-136 compliant system. The private communication system may be within a cell of a public communication system and the control information may then include identification information for the cell such as a public service profile for the cell.
In a further aspect of the present invention, a mobile terminal is provided for use with a private communication system which transmits control information on one of a plurality of control channel frequencies. The mobile terminal includes a means for receiving the control information including an identification of a candidate control channel frequency selected from the plurality of control channel frequencies on a first control channel frequency selected from the plurality of control channel frequencies. The mobile terminal further includes means for receiving control information from the private communication system on the candidate control channel frequency if control information from the private communication system is not being received on the first control channel frequency.
Accordingly, the present invention provides systems and methods allowing operation of a mobile terminal with both a public communication system and a private communication system even when the private communication system retunes its control channel while not in communication with the mobile terminal. This automatic selection of the new control channel for the private communication system is provided by transmitting to mobile terminals not only the control channel frequency (or frequencies) currently in use by the private communication system but also the candidate frequency (or frequencies) that the system will use for subsequent retunes. Accordingly, the mobile terminal is able to respond to retunes occurring while it is not in communication with the private communication system without the need for user intervention by using the saved candidate frequencies.